Concentric lay stranded conductors are well known within the electrical industry. Such conductors are customarily fabricated by stranding together a plurality of wires in concentric layers. The natural geometry of such a construction is that when round wires of the same diameter are used to form a stranded conductor, six wires fit around a single core wire, twelve wires fit around the layer of six wires, eighteen wires fit around the layer of twelve wires and so on with each successive layer containing six wires more than are contained in the layer around which they are being stranded. The number of individual wires contained in any conductor having "n" layers of wire and being so constructed is calculated by the algebraic equation X=3n.sup.2 +3n+1 where "X" represents the number of wires forming the conductor and "n" represents the number of layers being stranded about the central wire. The spaces between the individual wires are known as interstitial spaces and these spaces are inherent in conductors so manufactured.
One of the drawbacks with this type of conductor is the tendency for water and moisture to migrate throughout the length of the conductor by following these interstitial spacings. Attempts to prevent the flow of water or moisture, should it be introduced into such a conductor, have been made in many forms. Early attempts to solve the problem consisted of sealing the ends of the conductor at the point where it was joined to another conductor or at the point where it was terminated. A proper seal at the end of the length of the stranded conductor does indeed prevent moisture and/or water from flowing back into the conductor, however, problems with moisture in the conductor usually occur not from moisture or water entering the conductor through the end, but typically when moisture or water is allowed to enter the stranded structure through a flaw in the insulation or damage to the insulation which results in a hole in the insulating jacket of the conductor.
Other attempts to minimize the flow of moisture or water within the interstitial spaces of the stranded conductor came in the form of compacted or compressed stranded conductors. The stranded conductor itself was radially crushed in order to reduce the diameter of the conductor and to fill the interstitial spacing with metal from the individual wires themselves. The drawback to this method of blocking moisture is that even though some deformation of the individual wires does take place, and some of the interstitial spacing is filled, there is still substantial space present through which moisture and water will traverse.
A natural step toward correcting the problem of moisture flowing within this interstitial space consisted of filling the interstitial space with a foreign substance which physically prevented the flow of the moisture or water within the conductor structure. These compounds typically comprised some type of jelly base and a polyethylene filler material. At slightly elevated temperatures, this compound becomes fluid and viscous and can be applied to the stranded conductor as the conductor is being formed. The individual wires used to form the conductor are fed into an extrusion die where the moisture blocking compound is extruded onto and around each individual wire and, as the wires are stranded into the conductor, the interstitial space is filled with the jelly-like material. Upon cooling, the filler becomes very stable and immobile and does not tend to flow out of the interstitial spaces of the stranded conductor. That is to say, once the filling compound is applied within the interstitial spaces of the stranded conductor, it tends to remain in place. The problems encountered in applying such a filling compound revolve around precise metering of the compound into the interstitial spaces as the stranded conductor is being formed. If too much material is extruded into the conductor, the outer insulation will not fit properly. If too little compound is applied, the interstitial spaces will not be filled and therefore will allow moisture to flow within the conductor.
Another drawback to this method of applying the moisture blocking compound is that an extrusion head and an extrusion pump for applying the compound is required for every individual layer of wires used to form the concentric lay conductor. A seven strand conductor requires one extrusion pump and extrusion head. The second layer which forms a thirteen strand conductor requires an additional extrusion pump and extrusion head. The third layer which forms a nineteen strand conductor requires a third pump and extrusion head, and so on. The problems described above regarding the regulation of the volume of compound applied through an extrusion head are multiplied every time an additional extrusion pump and extrusion head is required within the conductor manufacturing system. Efforts to manufacture an acceptable moisture blocked conductor revolve around methods for uniform application of the moisture blocking compound to the concentric lay conductor.
Applications of moisture blocking compound to the interstitial spacing of concentric lay conductors is known within the industry. Evidence of this can be found in U.S. Pat. Nos. 3,607,487; 3,889,455; 4,105,485; 4,129,466; 4,435,613; 4,563,540; and 4,273,597.
U.S. Pat. No. 4,273,597 shows a method of strand filling. This invention deals with filling the interstitial spacing of a conductor not with a jelly-like substance as described above, but with a powder. This is accomplished by passing the strands through a fluidized powder bed, where the interstitial spacing is filled with the powder. The stranded conductor then exits the opposite end of the bed where an insulating layer is applied which prevents the powder from vacating the interstitial spacing of the conductor.
U.S. Pat. No. 4,563,540 describes a waterproof conductor which is constructed by flooding a waterproofing material among the individual conductors which make up the core of the stranded conductor. This flooded core is then wrapped with a plurality of different layers of shielding material which prevents the influx of moisture into the stranded conductor.
U.S. Pat. No. 4,435,613 describes an undersea conductor for transporting electrical power. This conductor is constructed of a plurality of layers of insulating material, with the core (or conducting portion) of the conductor being filled with an insulating layer of polyethylene. This polyethylene layer is contained by other rubber and plastic and epoxy compounds which produce a conductor having a waterproof construction.
U.S. Pat. No. 4,129,466 deals with a method for the application of the filling medium which is applied to a stranded conductor. This method comprises a chamber into which are passed individual wires that will be used to form the stranded conductor. These wires have a filling medium applied to them in the chamber. After the application of this filling medium, the conductor is passed through a chilling chamber where the filling medium is cooled and allowed to gel within the interstitial spaces. This method requires that the chamber containing the filling compound and the stranded conductor be both heated and pressurized. The heat applied to the chamber reduces the viscosity of the filling material, while the pressure assures introduction of the compound into the interstitial spaces of the stranded conductor.
U.S. Pat. No. 4,105,485 deals with the apparatus utilized in the '466 method patent previously discussed.
U.S. Pat. No. 3,889,455 deals with a method and apparatus for filling the interstitial spacing of the stranded conductor in a high temperature flooding tank. In this invention, the individual wires are fed into a tank containing the filling compound, the compound having been heated to allow it to become more viscous. The individual wires are actually stranded and closed within the confines of the flooding tank and the finished conductor is withdrawn from the opposite end of the flooding tank where it is passed through a cooling means. The disadvantages experienced here involve the practice of stranding the conductor beneath the surface of an elevated temperature moisture block pool. No access, either visual or mechanical, to the conductor manufacturing process is practical.
U.S. Pat. No. 3,607,487 provides a method whereby individual strands of wire are fed into a flooding tank which is supplied with heated filling compound by a pump and an injection means. The stranded conductor is withdrawn through the opposite end of the flooding tank and is wiped in a wiping die. The conductor is then wrapped in a core wrapper. The wrapped core is then passed through a binder where it is bound. The bound, wrapped core is then passed through a cooler which sets the gel. The above described process is repeated through another flooding tank, another cooler, another binding machine, another flooding tank, another extruder, another cooling trough, and is eventually withdrawn from the end of the manufacturing line as a product having a plurality of layers of moisture blocking compound which protects the conductor core of the conductor. The disadvantages here comprise a complex manufacturing line whereby moisture blocking compound is applied at three different locations, each location having to be meticulously monitored and controlled in order for a proper conductor construction to be obtained.
It can be readily seen from the above referenced methods and apparatuses that moisture blocked conductor is known and is being manufactured in the industry today. It can also be recognized that there are major problems concerning the uniform application of the moisture blocking compound during the fabrication of the stranded conductor.